The 91.175(h) table
When a takeoff or landing minimum is published as RVR and RVR is not reported for that runway, 14 CFR 91.175(h)(1) says the comparable ground visibility from the table in (h)(2) becomes the minimum — except for Category II or III minimums. The table is:
1,600 ft → 1/4 SM · 2,400 ft → 1/2 SM · 3,200 ft → 5/8 SM
4,000 ft → 3/4 SM · 4,500 ft → 7/8 SM · 5,000 ft → 1 SM · 6,000 ft → 1 1/4 SM
These are comparable values for minima, not a unit conversion. 2,400 ft is not physically one-half of a statute mile (that would be 2,640 ft). The gaps are uneven, which is why the rule does not interpolate. The FAA AIM reprints the same table.
Values between rows
The regulation itself has no comparable visibility for an RVR that is not one of those seven numbers. The Terminal Procedures Publication legend describes a practice for other conversions: use the next higher listed RVR, and do not interpolate. Its example is 4,800 RVR → 5,000 RVR → 1 SM. This page will show that next-higher row when you enter a between-row value, and will label it as the legend practice rather than an exact 91.175 match. The starred 91.175 rows remain the ones to use for takeoff and landing minimums when RVR is not reported.
Sources
14 CFR 91.175(h), Comparable values of RVR and ground visibility; FAA Aeronautical Information Manual (same table); FAA Terminal Procedures Publication legend, Comparable Values of RVR and Visibility (next-higher-RVR practice). For a raw METAR that may include RVR, see the METAR decoder.
Frequently asked questions
What visibility is 2,400 RVR?
One-half statute mile, by the 14 CFR 91.175(h)(2) table. That is the comparable ground-visibility minimum when 2,400 RVR is prescribed but RVR is not reported, except for Category II or III.
Can I interpolate 4,800 RVR to something between 7/8 and 1 mile?
No. The regulation does not interpolate. The TPP legend practice is to take the next higher listed RVR, so 4,800 becomes 5,000 RVR, which is 1 statute mile. This page shows that and labels it as the legend practice.
Does this apply to Category II or III minimums?
No. 91.175(h)(1) excludes Category II and III. Use the charted CAT II/III procedure and the operating rules that go with it.